Biological microscopes are instruments used for observing microbes, cells, bacteria, tissue culture, suspended substances, and sediments in medical and health institutions, institutions of higher education, and research institutes, and can be used for continuously observing reproduction and fission processes of cells and bacteria and the like in culture media. The biological microscopes are widely applied in the fields such as cytology, parasitology, oncology, immunology, genetic engineering, industrial microbiology, and phytology.
Generally, the sizes of animal cells and plant cells are between 10 μm and 100 μm, the size of a relatively small erythrocyte is less than 6 μm, and a platelet and a fungus have even smaller sizes, which may be less than 3 μm. To observe a tangible component in blood, urine, or other body fluids clearly, a microscope is required to have sufficient resolution. When observing a small target such as an erythrocyte, a leukocyte, or a platelet, a microscope is required to have resolution less than 1 μm. However, to observe a big target of a size greater than 10 μm, resolution is required to be less than 3 μm. (Note: resolution of a microscope refers to a capability of clearly distinguishing, by using the microscope, a minimum distance between two object points; and when the size of a cell is three times as large as minimum resolution, the number of clearly recognizable pixel points is seven, and in this case, only one pixel point is observable inside a cell. As is shown in FIG. 1, if the cell size is three times less than the minimum resolution, none structure inside the cell can be clearly observed.)
Common object lenses for biological microscopes used for cell analysis have three specifications, which are 10×, 20×, and 30×, respectively, and generally numerical apertures (NAs) corresponding to the three specifications are 0.25, 0.45, and 0.65, respectively. Therefore, resolution of a microscope can be calculated by:d=λ/NA,
wherein λ is a wavelength of a light source, and NA is a numerical aperture of an object lens. Assuming that an average wavelength λ of a light source is 0.6 μm, resolution of different object lenses can be obtained as shown in Table 1 below.
TABLE 1Magnification of object lens102040Numerical aperture (NA)0.250.450.65Resolution (d = λ/NA)2.4 μm1.33 μm0.923 μm
As can be seen, to observe a small target such as a platelet and a fungus clearly (a microscope is required to have resolution less than 1 μm), an object lens whose numerical aperture is greater than 0.6 needs to be provided; a method of localizing with a low magnification lens (e.g. 10× lens) and tracking and identifying with a high magnification lens (e.g. 40× lens) is generally used for implementation (refer to Patent No. 201110315831.4 for details). However, during an observing process in which this method is used, object lenses need to be adjusted for switching between high and low magnification, operations are complex, and a mechanical error occurs easily, leading to an offset in positioning between high and low magnification and causing inaccurate positioning. In addition, due to the fluidity of a sample, the position of a target that is localized in low magnification may change when low magnification is switched to high magnification for observation, or even deviates from a field of view causing missed identification.
In view of the problem of inaccurate positioning because lenses with high and low magnification need to be switched during a target screening process in the prior art, an effective solution is yet to be proposed at present.